Jackson visits Leroy for a special reason. Meanwhile, a couple, out in a park in freezing weather, finds the body of a Marine first lieutenant, barefoot and thinly clothed. Gibbs and company figure it out and elicit a confession.

Diane, the mutual ex-wife of Gibbs and Fornell, gets help from them; Victor, her current husband, has disappeared; they and the gang find the pieces, put them together, find the bad guys, and deal with them. Diane gives Gibbs a watch.

Within 30 miles of Washington, DC, eight kids in Navy and Marine Corps families have suddenly become badly ill, requiring treatment in ERs and ICUs; Gibbs and company investigate; Carol, of the Naval Medical-research Center, joins the effort; immediately the number of sick kids grows to 14, then 19, 23, 28, and 34, and none of them responds to treatment. Duckie announces the first fatality, who was a veteran of Vietnam, then he describes the cause of death and the treatability in the kids. Abby says that she has misidentified the particular nature of the illness, so she, Jimmy, and Carol keep working, and they eventually nail it. Abby gives Jimmy a lecture about kindness, optimism, and parenthood. Then the gang identify an international traveler who in many places has recently encountered many kids and the fatality. Meanwhile Vance has three tense chats with his estranged father-in-law and then a nicer one.

During a hit-and-run investigation, Palmer tries to prevent a stranger from jumping off the ledge of a building.

Investigating the death of an intruder at Quantico, Jimmy and Kasie are exposed to a deadly bio-toxin. The NCIS team calls on Doctor Carol Wilson for help as they race to find the antidote. Torres tries to distract Jimmy's daughter, who came to spend the day with her dad.